What controls the mechanical properties of shale rocks? – Part II: Brittleness

Rybacki, E.; Meier, T.; Dresen, Georg

doi:10.1016/j.petrol.2016.02.022

Cited by 280 publications

(144 citation statements)

References 47 publications

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“…The presence of water also accelerates the rate of fracture permeability reduction, in particular for clay-rich shales (Akrad et al, 2011;Alramahi and Sundberg, 2012;Guo and Liu, 2012;Kassis and Sondergeld, 2010;LaFollette and Carman, 2010;Liu and Sharma, 2005;Morales p [4] et Pedlow and Sharma, 2014;Reinicke et al, 2010;Stegent et al, 2010;Volk et al, 1981;Zhang et al, 2013;Zhang et al, 2015;Zhang et al, 2016b). The brittleness of shales, which correlates to some extent with their composition and elastic properties (Rybacki et al, 2016;Zhang et al, 2016a), may be also relevant for the long-term fracture healing rate. Fast healing is expected for more ductile shales by rapid proppant embedment or enhanced creep of contact areas in unpropped fractures.…”

Section: Introductionmentioning

confidence: 99%

Creep of Posidonia Shale at Elevated Pressure and Temperature

et al. 2017

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The economic production of gas and oil from shales requires repeated hydraulic fracturing operations to stimulate these tight reservoir rocks. Besides simple depletion, the often observed decay of production rate with time may arise from creep-induced fracture closure.We examined experimentally the creep behavior of an immature carbonate-rich Posidonia shale, subjected to constant stress conditions at temperatures between 50° and 200°C and confining pressures of 50 to 200 MPa, simulating elevated in-situ depth conditions. Samples showed transient creep in the semibrittle regime with high deformation rates at high differential stress, high temperature, and low confinement. Strain was mainly accommodated by deformation of the weak organic matter and phyllosilicates and by pore space reduction.The primary decelerating creep phase observed at relatively low stress can be described by an empirical power law relation between strain and time, where the fitted parameters vary with temperature, pressure and stress. Our results suggest that healing of hydraulic fractures at low stresses by creep-induced proppant embedment is unlikely within a creep period of several years. At higher differential stress, as may be expected in situ at contact areas due to stress concentrations, the shale showed secondary creep, followed by tertiary creep until failure. In this regime, microcrack propagation and coalescence may be assisted by stress corrosion.Secondary creep rates were also described by a power law, predicting faster fracture closure rates than for primary creep, likely contributing to production rate decline. Comparison of our data with published primary creep data on other shales suggest that the long-term creepbehavior of shales can be correlated to their brittleness estimated from composition. Low creep strain is supported by a high fraction of strong minerals that can build up a load-bearing framework.

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Section: Introductionmentioning

confidence: 99%

Creep of Posidonia Shale at Elevated Pressure and Temperature

et al. 2017

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“…The Brazilian test is an ISRM recommended method for measurement of the tensile strength of rocks (ISRM 1978), it is an easy and common method for determining the tensile strength of rock. Tensile strength is calculated in this test by using an equation, which assumes isotropic material properties, as shown in Equation (1). The principal tensile stress, in particular at the rock disc center where a crack initiates, should be known when using Equation (1) to obtain the tensile strength of rock samples.…”

Section: Anisotropic Brazilian Testmentioning

confidence: 99%

“…Using Equation (2), the dynamic tensile strength of the shale sample was calculated, as listed in Table 2. At the same time, we also obtained the tensile strength using Equation (1). When using Equation (1) to calculate the tensile strength, the structural anisotropy of shale is not considered, and the results are different from the results from Equation (2).…”

Section: Tensile Strength Of Anisotropic Shalementioning

confidence: 99%

“…Platy minerals such as clays have a tendency to be aligned in a parallel orientation, and form geologic discontinuities. These geologic discontinuities have a major influence on the mechanical behaviors in the oil-gas field, including hydraulic fracturing, borehole stability assessment, evaluation of stable mud weight windows for rock drilling, as well as other rock engineering problems [1,2]. The tensile strength of shale can be strongly affected by the bedding planes, which often results in anisotropy.…”

Section: Introductionmentioning

confidence: 99%

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Brazilian Test for Tensile Failure of Anisotropic Shale under Different Strain Rates at Quasi-static Loading

Wang

et al. 2017

Energies

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Shale formations show obvious anisotropic characteristics in their mechanical properties due to pronounced bedding planes and natural fractures. This anisotropic behavior generally creates complex fracturing networks and is crucial to gas shale stimulation. Although much research has been done to study the anisotropic compression behaviors of shale with static and quasi-static strain rates, there are limited investigations addressing the anisotropic tensile behaviors of shale at quasi-static strain rate. In this work, the anisotropic tensile behaviors of Longmaxi shales were studied systematically at different strain rates from 10 −5 to 10 −2 s −1 by performing Brazilian splitting tests. Testing results reveal the tensile strength anisotropy, rate dependency, and the stimulated fracture pattern morphology. The results show that the orientation between the applied force and bedding direction has an obvious effect on the tensile strength and fracture pattern. The rate dependency of shale under different loading rates is different for shale samples with various orientations. It was suggested that a complex tensile fracture pattern can be easily formed when using a high loading rate. The result sheds light on how to stimulate a complex fracturing network during field hydraulic fracturing treatment.

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“…Shale gas extraction requires the use of hydraulic fracturing technology [4], a technology that can benefit from understanding the mechanical properties [5,6]. Shales generally exhibit low porosity and permeability [7][8][9][10]. Better grasp of these characteristics helps to improve the fracturing process.…”

Section: Introductionmentioning

confidence: 99%

Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

et al. 2016

Energies

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Abstract:With the exploitation of shale gas booming all over the world, more and more studies are focused on the core technology, hydraulic fracturing, to improve commercial exploitation. Shale gas resources in China are enormous. In this research, a series of tests were carried out with samples of black organic-rich shale from the Lower Silurian Longmaxi formation, south China. Samples were drilled from different directions and were subjected to uniaxial and triaxial condition with various confining pressures, aiming at studying its rock mechanics properties, so as to provide basis for research and breakthrough of hydraulic fracturing technology. According to the results of the study, the development and distribution of shale's bedding planes significantly impact its mechanical properties. Shale samples show obvious brittle characteristics under low confining pressure, and its mechanical behavior begins to transform from brittle to plastic characteristics with increasing confining pressure. Shale samples with different inclinations (β) have different sensitivities to the confining pressure. As a result, samples with 45 • inclinations (β) are least sensitive. The strength of bedding planes is significantly lower than that of shale matrix, and tensile failure and shear failure generally tend to occur along the bedding planes. When hydraulic fracturing was conducted in shale formation with depth less than 2.25 km, corresponding to original in-situ of 60 MPa, cracks will preferably occur at first along the inclination (β) angle of 45 • from the maximum principal stress, and the failure mode is most likely to be shear failure without volumetric strain. And, different modes of failure will occur at different locations in the reservoir, depending on the orientation of bedding inclined from the principle stress, which can probably explain the phenomenon why there are fractures along and cross the bedding planes during hydraulic fracturing treatment. When hydraulic fracturing was conducted in shale formation with depth greater than 2.25 km, hydraulic fractures may not crack along the bedding surfaces to some extent.

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What controls the mechanical properties of shale rocks? – Part II: Brittleness

Cited by 280 publications

References 47 publications

Creep of Posidonia Shale at Elevated Pressure and Temperature

Creep of Posidonia Shale at Elevated Pressure and Temperature

Brazilian Test for Tensile Failure of Anisotropic Shale under Different Strain Rates at Quasi-static Loading

Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

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